With our current repositories you can not downgrade. If you have had the package installed in past than the package probably still exists in your cache, unless you did equo cleanup. I believe there is away to install than, I just don't know the commands as I have never done it. Check equo --help
A member recently started up a community repository that he calls Frozen Repository, which allows users to get older packages from time of release. See viewtopic.php?f=76&t=28837
He is starting with Sabayon 10 so prior to that is a no go. His version of xorg and drivers would be older than current tho so you may want to check into that.